This invention is in the field of atomic force microscopes and micro-cantilevers. This invention relates generally to a microcantilever having a temperature compensating piezoresistive strain sensor and integrated heater-thermometer. This invention also relates to methods of using such a cantilever in the fields of thermodynamic measurements and chemical/biochemical sensing.
Since the invention of the atomic force microscope (AFM), microcantilevers have become one of the most frequently used microelectromechanical systems (MEMS) devices with applications ranging from scanning probe microscopy to bio/chemical sensing. Microcantilevers are often functionalized by introducing current traces, piezoresistive or piezoelectric materials to realize specific applications. Heatable microcantilevers having either doped single-crystalline or polycrystalline silicon or patterned metal traces allow current flow so that they can be heated by means of resistive (Joule) heating. This heating in microcantilevers can be used for bimorph actuation, thermomechanical data storage, thermal displacement sensing in contact and tapping modes, novel nano-material synthesis, nanoscale thermal analysis and nanoscale thermal manufacturing. Recently, cantilever type micro-hotplates have been reported as an alternative platform for calorimetry. A cantilever type micro-hotplate fabricated based on silicon technology has been introduced and several microcantilever hotplates were designed, fabricated, and characterized to investigate response time and temperature uniformity.
Microcantilevers with integrated piezoresistive strain sensors are mainly used to replace optical deflection sensing but are also employed in various sensing applications such as gas flow sensing, acceleration sensing, microjet measurements and bio/chemical sensing. Especially as bio/chemical sensors, piezoresistive microcantilevers are often prepared with a selective coating sensitive to a specific analyte. Analyte adsorption induces static deflection by creating a surface stress, and thus embedded piezoresistors can measure analyte adsorption.
Microcantilevers having both resistive heaters and piezoresistors can offer simultaneous heating and deflection sensing. These hybrid types have been used as multi-functional scanning probes in thermomechanical data storage. Similarly, microcantilevers with the ability of independent heating and sensing operation that have high sensitivity to surface stress could be used for a variety of sensor applications. One example would be calorimetry of a material adhered to the cantilever surface. Chemical processes such as melting and evaporation and chemical reactions between substances could be triggered by the heaters while the changes in the surface stresses on the cantilever are monitored and can give information about the material or reaction properties. Other examples include biochemical sensing, where one might wish to interrogate the temperature-dependence of biochemical binding to a microcantilever.
One strategy for suppressing unwanted signals, such as temperature drift, in piezoresistive cantilever sensors is to fabricate cantilever pairs for a differential measurement. Two microcantilevers with identical piezoresistive strain gauges can be arrayed closely and interfaced in a Wheatstone bridge to cancel temperature drift with the assumption they have the same temperature progression. However, this approach would not be appropriate for cases in which the two cantilevers experience different temperatures. This could be the case when a reactive coating modifies the thermal properties of one cantilever. Temperature deviations between the two devices can also be caused by the system environment, e.g. by thermal gradients due to gas flow directions. On-chip temperature compensation for piezoresistive cantilever sensors has been demonstrated, but these cantilevers did not have integrated heater-thermometers. Furthermore, all previous approaches to on-chip temperature compensation use the principle of a Wheatstone bridge circuit on the cantilever, a method which assumes unidirectional, equal stress in all resistors. However, for chemical sensing, in which a reactive layer causes a surface stress on the silicon surface, the stress distribution in the cantilever is complex and three-dimensional. Therefore, it is favorable to incorporate independent sensors for stress and temperature in the cantilever to correct the effect of thermal variations on the mechanical signal.